Parasitic diseases are among the leading causes of death globally, collectively second only to cardiovascular diseases. Malaria is one of the most significant parasitic diseases resulting in an estimated 500 million new infections and 2 millions deaths annually, predominantly in sub-Saharan Africa and Asia. It is caused by members of the genus Plasmodium, in particular by Plasmodium falciparum, which is the principal malarial protozoan parasite in humans. Similarly, leishmaniasis is a tropical/sub-tropical parasitic disease, caused by members of the genus Leishmania, which infects over two million people annually. Toxoplasma parasites, the causative agents of toxoplasmosis, can cause serious opportunistic infections in immune-compromised individuals such as those with AIDS, or in people undergoing chemotherapy or organ transplants. Trypanosomiasis is another widespread parasitic infection, caused by Trypanosoma cruzi and Trypanosoma brucei which are transmitted by insect vectors. Likewise, schistosomiasis is a parasitic disease caused by several species of trematodes, a parasitic worm of the genus Schistosoma. Together parasitic diseases pose a significant health and economic burdens, particularly in underdeveloped regions. There is currently no effective vaccine targeted to any human parasitic disease, therefore anti-parasitic drugs continue to be crucial for the treatment of the diseases. Unfortunately, where drugs are available, they are under increasing threat of failure owing to drug-resistant parasites. Intense research efforts are underway to discover and develop new drugs to treat neglected diseases and to address the growing problem of parasite resistance (Dondorp A M et al, N. Engl. J. Med. 2009, 361, 455-467).
Histone deacetylase (HDAC) inhibitor drugs, originally targeted for cancer use, are now being investigated to target a range of parasitic diseases (Andrews K T et al, Immunology and Cell Biology 2012, 90, 66-77).
Histone-modifying enzymes are crucial for modulating cell chromatin structure and gene expression in eukaryotic organisms. Histone deacetylases (HDACs) and histone acetyl transferases (HATs) determine the pattern of histone acetylation, which together with other dynamic sequential post-translational modifications might represent a ‘code’ that can be recognised by non-histone proteins forming complexes involved in the regulation of gene expression. This and the ability of histone deacetylases (HDACs) to also modify non-histonic substrates and participate in multi-protein complexes contributes to the regulation of gene transcription, cell cycle progression and differentiation, genome stability and stress responses.
HDAC inhibitors cause the induction of differentiation, growth arrest and/or apoptosis in a broad spectrum of transformed cells in culture and tumours in animals, including both haematological cancers and solid tumours. These inhibitory effects are believed to be caused, in part, by accumulation of acetylated proteins, such as nucleosomal histones, which appear to play a major role in regulation of gene transcription.
HDACs have been identified in all the major human parasitic pathogens. The potential of HDAC inhibitors as antiparasitic agents was first realized over a decade ago when the cyclic tetrapeptide apicidin was found to have broad spectrum anti-parasitic activity (Darkin-Rattray S J et al, Proc Natl Acad Sci USA 1996, 93, 13143-13147). Pan inhibitors of both class I and II HDACs, such as the cyclic tetrapeptide apicidin and the hydroxamate tricostatin A (TSA), have potent antimalarial activity in vitro (Darkin-Rattray S J et al). These compounds cause hyperacetylation of P. falciparum histones, indicating inhibition of one or more PfHDACs. However, both apicidin and TSA suffer from metabolic instability and neither is parasite-selective, so without modifications that overcome these problems, both are unsuitable as antiparasitic drugs. In contrast to TSA, SAHA has less potent activity against P. falciparum (IC50˜100-300 nM), but somewhat improved parasite-specific selectivity (Dow G S et al, Antimicrob Agents Chemother 2008, 52, 3467-3477). Despite its clinical use for cancer, the in vivo efficacy of SAHA against Plasmodium parasites in murine malaria models has not yet been reported.
Several hydroxamic acid-based HDAC inhibitor analogues have been described with better potency against P. falciparum parasites in vitro than SAHA and in some case much better selectivity. HDAC inhibitor analogues screened in those studies included compounds based on L-cysteine, 2-aminosuberic acid (Andrews K T et al, Antimicrob Agents Chemother 2008, 52, 1454-1461), triazolylphenyl (Chen Y et al, J Med Chem 2008, 51, 3437-3448), compounds with cinnamate or non-steroidal antinflammatory components (Wheatley N C et al, Bioorg Med Chem Lett 2010, 20, 7080-7084), and a panel of 50 phenyl-thiazolyl hydroxamate based HDAC inhibitor analogues (Dow G S et al, Antimicrob Agents Chemother 2008, 52, 3467-3477). The latter study identified compound WR301801 which was found to hyperacetylate P. falciparum histones and inhibit P. falciparum deacetylase activity in nuclear extracts. However, orally administered WR301801 was not able to cure mice unless co-administered with sub-curative doses of the anti-malarial drug chloroquinine SB939, a hydroxamic acid that is a pan inhibitor of mammalian HDACs and shows antitumoral activity, was reported to be a potent inhibitor of the growth of P. falciparum in vitro causing hyperacetylation of parasite histone and non-histone proteins (Sumanadasa S D M et al, Antimicrob. Agents Chemother. 2012, 56, 3849-3856). When SB939 was administered orally in an in vivo murine model of cerebral malaria it significantly inhibited P. Berghei ANKA parasite growth, preventing development of cerebral malaria-like symptoms.
WO2006/017214 describes hydroxamic acid derivatives that are inhibitors of histone deacetylases, useful for treating cellular proliferative diseases and also having antiprotozoal properties. WO2008/019025 relates to isoform-selective HDAC inhibitors useful for treating cancer, neurological diseases and malaria. WO2009/042646 relates to multifunctional molecules wherein one pharmacophore is capable of inhibiting zinc-binding enzymes (e.g. HDAC) and one pharmacophore is capable of inhibiting a different cellular function involved in aberrant proliferation, differentiation or survival of cells. Such molecules are disclosed for the treatment of many disorders, including protozoal infections. However, all these compounds have low specificity.
These findings demonstrate the potential of HDAC inhibitors as potential anti-parasitic drugs. However, to progress HDAC inhibitors as drugs for parasitic diseases, a high level of potency and selectivity for parasites versus host cells is essential. Further, improved pharmacokinetic profiles are needed to accommodate the unique challenges facing the application of drugs for developing-world diseases, such as oral efficacy and complementary profiles with combination drugs, for single infections (as in the case of malaria), or poly-parasitism and polymicrobial infections (for example, HIV and parasite co-infection).
WO 2006/061638 describes HDAC inhibitors useful for treating cellular proliferative disease, with improved pharmacokinetic properties.
In the present invention, further investigation of this class of compounds lead to the identification and selection of amides as a zinc binding moiety and substitution patterns that enhanced the selective anti-parasite activities. These features allow for highly selective killing action against parasites rather than normal host cells. In particular, compounds of the invention are HDAC inhibitors that incorporate an amide as zinc-binding group and selectively suppress the growth of Plasmodium falciparum at a lower concentration than the concentration required for the inhibition of the growth of mammalian cells in culture.